The combustion of a fossil fuel in fossil fuel-fired power plants for generating electricity produces a carbon dioxide-containing flue gas. To prevent or reduce carbon dioxide emissions, carbon dioxide has to be separated from the flue gases. Various methods are generally known for separating carbon dioxide from a gas mixture.
In particular the absorption-desorption method is commonly used to separate carbon dioxide from a flue gas after a combustion process (Post-Combustion CO2 Capture, Post Cap). On a large industrial scale, in such a method carbon dioxide is scrubbed out of the flue gas with an absorbent at around atmospheric pressure. In a conventional absorption-desorption process the flue gas is brought into contact with a selective solvent as absorbent in an absorption column, the carbon dioxide being absorbed by the solvent. The solvent enriched with carbon dioxide is passed into a desorption column in order to separate the carbon dioxide and regenerate the solvent. The solvent is heated (possibly also depressurized), wherein carbon dioxide is desorbed again and a regenerated solvent is formed. The regenerated solvent is again passed to the absorber column, where it may again absorb carbon dioxide from the carbon dioxide-containing flue gas. Common absorbents display good selectivity and an elevated capacity for the carbon dioxide to be separated. Particularly suitable absorbents are those which are based on amines, such as for example monoethanolamine. In the chemical industry too, amine solutions are generally used as absorbents.
The Pre-Combustion CO2 Capture (Pre Cap) method, in which a CO shift and physical carbon dioxide scrubbing proceed under elevated pressure (IGCC concept), is commonly used to separate carbon dioxide prior to the combustion process.
When using standard natural gas as combustion fuel for a gas turbine, carbon dioxide is separated from the flue gas preferably after combustion in a standard gas turbine with premix combustion. Standard natural gas is in this case distinguished by a correspondingly high calorific value and a low proportion of inert gases (for example nitrogen or carbon dioxide).
On the other hand, in the case of a “lean” natural gas, which contains an elevated proportion of inert gases, such as for example natural gas with a high carbon dioxide content (e.g. 30 to 70% carbon dioxide content), said gas has first to be conditioned for combustion in a standard gas turbine.
To this end high grade natural gas may be admixed with the lean natural gas, for example, and thus enriched to a methane content which allows combustion in a standard gas turbine. It is also possible to allow combustion of a lean natural gas in a standard gas turbine by means of combustion with oxygen and subsequent condensation of water. However, these variants require high expenditure and additional gases.
It is alternatively possible to adapt the gas turbine specifically to low-calorific, lean natural gas. However, this may entail enormous development expenditure, depending on machine model.